Recently, developments in video signal input/output techniques for a variety of video media have been demanding a video screen of higher quality. A clamping circuit for fixing a peak value, such as a minimum or maximum value of an input signal, to a constant power source level is used primarily in cases in which a reference direct current (DC) level is lost due to distortion of the video signal or the input signal of a television, etc. Since a conventional video signal clamping circuit typically has a clamping level at an output terminal that varies with the peak level of an input video signal, the inconstant peak level acts an a unstable factor in reproducing the video signal.
FIG. 1 is a block diagram of such a conventional video signal clamping circuit. The circuit of FIG. 1 is constructed such that if a video signal is supplied, an output signal having a constant peak level is formed by external condenser 11 and clamper 10. FIG. 2 is a circuit diagram of the conventional clamping circuit of FIG. 1. Referring to FIGS. 1 and 2, transistors Q.sub.B and Q.sub.C have the same characteristics, and resistors R.sub.D and R.sub.E have the same resistance values. Since transistors Q.sub.B and Q.sub.C operate at the same base-to-emitter voltage V.sub.BE, respective base currents flowing into the respective bases of transistors Q.sub.B and Q.sub.C are the same, and likewise, respective collector currents flowing into respective collectors thereof are the same, thereby forming a current mirror. One terminal of resistor R.sub.B is connected to the base and collector of transistor Q.sub.B. The other terminal of resistor R.sub.B is connected to resistor R.sub.A and the base of transistor Q.sub.A. The input video signal is transmitted to node N1 through external condenser (CB) 11. The emitter of transistor QD is connected to constant current source Is at output node N2 connected to output terminal VOUT.
In operation, since transistors Q.sub.B and Q.sub.C have a current mirror structure, if resistors R.sub.D and R.sub.E have the same resistance values, then the base currents of transistors Q.sub.B and Q.sub.C are the same, and the collector currents thereof are the same. Assuming that each collector current flowing into the collectors of the transistors Q.sub.B and Q.sub.C is I.sub.B, I.sub.B can be represented by the following equation (1): ##EQU1## where V.sub.BE (Q.sub.B) is the voltage between the base and emitter of transistor Q.sub.B. Since elements V.sub.CC, V.sub.BE (Q.sub.B), R.sub.A, R.sub.B and R.sub.D have constant values, current I.sub.B can be treated as a constant.
A voltage V1 across the base node of transistor Q.sub.A can be obtained by substituting Ohm's law (V=IR) for I.sub.B given from equation (1): ##EQU2## A voltage at node N1 directly connected to the input terminal is obtained by subtracting the base-to-emitter voltage V.sub.BE (Q.sub.A) of transistor Q.sub.A from voltage V1 across the base node of transistor Q.sub.A. That is: EQU V1-V.sub.BE (Q.sub.A) (3)
When there is no input signal, since the voltage at node N1 given from expression (3) is a voltage across the base of transistor Q.sub.D, a voltage at output node N2 connected to the emitter of transistor Q.sub.D is obtained by adding V.sub.BE (Q.sub.D) to V1-V.sub.BE (Q.sub.A). That is, the output voltage is as follows: EQU V1-V.sub.BE (Q.sub.A)+V.sub.BE (Q.sub.D) (4)
FIG. 3 is a waveform chart illustrating an operation of the video signal clamping circuit of FIG. 2. A clamping voltage of the output terminal for the input video signal having a varied peak level varies by a varying difference.
If the input signal level received to the input terminal is lower than the voltage at node N1 connected to the input terminal, EQU V.sub.IN &lt;V1-V.sub.BE (Q.sub.A), (5)
then the current of transistor Q.sub.A with the emitter connected to node N1 is increased and thus V.sub.BE (Q.sub.A) is increased. The voltage level of V1-V.sub.BE (Q.sub.A) of expression (3) is lowered. The peak level of the output signal is clamped to a voltage determined by equation (6). Since V1-V.sub.BE (Q.sub.A) is lowered, the clamping level is lowered . EQU V.sub.OUT =V1-V.sub.BE (Q.sub.A)+V.sub.BE (QD) (6)
Therefore, if the peak level of the input video signal varies, then the conventional video signal clamping circuit reflects the varied peak level in the clamped output signal. Current I.sub.A is divided into I.sub.B and I.sub.C by node N1 connected to the input terminal as can be represented by the following equation (7): EQU I.sub.A =I.sub.B +I.sub.C ( 7)
If the peak level of the input video signal varies, then I.sub.C also varies. Since I.sub.B calculated by equation (1) can be treated as a constant, IA varies in proportion to I.sub.C.
The base-to-emitter voltage V.sub.BE (Q.sub.A) of transistor Q.sub.A is determined by emitter current I.sub.A of transistor Q.sub.A. If I.sub.A varies, then V.sub.BE (Q.sub.A) varies. Therefore, clamping level V.sub.OUT represented by equation (6) varies according to V.sub.BE (Q.sub.A) since V1 and V.sub.BE (Q.sub.A) have a constant value.
That is, the clamping level V.sub.OUT of the output terminal varies with V.sub.BE (Q.sub.A), V.sub.BE (Q.sub.A) varies with I.sub.A, I.sub.A varies with I.sub.C, and I.sub.C is proportional to a variation in the peak level of the input video signal. As a result, clamping level V.sub.OUT varies according to variations in the peak level of the input video signal. Hence, the desired results can not be obtained in reproducing the video signal.